Synthesis of (E)-Alkenyl Iodides via (E)-[2′-(Tributyltin)alkenyl]dialkylboranes with Iodine

摘要

Alkenyl iodides, which are important starting materials in organic synthesis, are usually prepared by sequential reactions of alkynes, i.e., hydrometalations of various metal hydrides to alkynes followed by demetalations with iodine. The hydrometalations of alkynes is convenient to prepare 1-iodo-1-alkenes in anti-Markovnikov fashion, but internal alkenyl iodides are scarcely obtained by these methods. For this purpose, the haloboration of B-bromo- or B-Iodo-9-borabicyclo[3.3,1]nonane (BBN) has been developed, but only terminal alkynes can be haloborated by B-bromo or B-iodo-9-BBN; therefore, only 2-bromo- or 2-iodo-1-alkenes can be prepared by this method. Other methods, such as the stereoselective synthesis of internal alkenyl iodides by the hydroiodina-tions of alkynes via in situ generation of HI or its equivalent have also been reported. But those methods could give satisfactory results only when the alkynes used are terminal or symmetric internal alkynes. In the case of unsymmetric internal alkynes, those reactions would give a mixture of two isomers owning to the low regio-selectivity of the reaction. Thus, those methods are also not suitable for preparing the internal alkenyl iodides I with two different alkyl groups (R ≠ R′, R′ ≠ H) on the carbon-carbon bond.